Theories from historical migration research: Social psychology or economics?

There exists no generally accepted, coherent theory to explain migration either as a contemporary or as a historical phenomenon. Historians and economists have, however, developed several partial models, particularly with reference to mass emigration from Europe to the USA. These models stress independent variables such as wage differentials and employment rates, which can be classified as economic factors. Several researchers have argued that social psychologic factors, such as the rural-industrial barrier and the multiplier effect, must be given equal weight if the dynamics of mass migration are to be understood. Methodologic issues seem to be crucial for theory development, since researchers that-stress economic factors tend to use aggregated data series, whereas those who put more weight OH explanations from social psychology have worked from data on the individual level.